1. Field of the Invention
The present invention relates to direct-view-type display devices; such as those of a transmission type and a reflection type, a projection type display device; a liquid crystal display device used in various information processing devices; and a method for producing the same. More specifically, the present invention relates to a liquid crystal display device in which spacers are selectively dispersed between two substrates with a liquid crystal layer interposed therebetween so as to control a cell gap.
2. Description of the Related Art
Liquid crystal display devices have a structure in which a pair of substrates, at least one of which is transparent, are provided with a predetermined gap and liquid crystal is injected between the substrates. In order to keep this predetermined gap, for example, spacers are dispersed on the surface of the substrate on a liquid crystal layer side, and then the pair of substrates are attached to each other. Because of this structure, a cell gap is kept almost equal to a size of the spacers. Conventionally, the spacers are dispersed almost over the entire surface of the substrate on the liquid crystal layer side.
Such liquid crystal display devices have the following problems: When liquid crystal is injected or after the substrates are attached to each other to construct a panel, the spacers move due to the vibration, change in temperature, or the like, to cause inconsistencies in color resulting from inconsistencies in cell gap, and the spacers damage alignment films while moving to cause inconsistencies in display.
In addition, when spacers are dispersed on the substrate so as to be placed in regions contributing to a display (i.e., pixels), since the spacers exhibit optical anisotropy different from that of liquid crystal, light leaks through the spacers resulting in decreased contrast.
Japanese Laid-Open Patent Publication No. 6-95127 discloses the following method for fixing spacers on an alignment film: in a step for spraying spacers on an alignment film containing a solvent, the spacers are heated by high frequency radiation before the spacers reach the alignment film; the heated spacers are partially varied in the alignment film and fixed at the positions because the heat of the spacers evaporates the solvent of the alignment film and cures the alignment film. Then the substrate is subject to a heat treatment to cure the alignment film completely. The spacers may be coated with a thermoplastic polymer. Since the spacers are allowed to adhere to the entire surface of the alignment film formed on an electrode substrate, this method has a problem in that light leakage through spacers present in the pixel regions cause a decrease in contrast.
In order to solve the above-mentioned problem, a method for placing spacers only in regions other than pixels has been suggested. Specifically, a method utilizing a photosensitive thermoplastic resin as described in Japanese Laid-Open Patent Publication No. 2-308224; a method utilizing static electricity as described in Japanese Laid-Open Patent Publication No. 4-321013; a method for placing spacers in a black mask as described in Japanese Laid-Open Patent Publication No. 6-43468, have already been suggested.
According to the method utilizing a photosensitive thermoplastic resin as disclosed in Japanese Laid-Open Patent Publication No. 2-308224; before an alignment film is formed on a substrate, a photosensitive thermoplastic resin film is formed on the substrate and patterned so that the patterned photosensitive resin film is formed only in regions other than pixels, and spacers are dispersed over the substrate. The substrate is heated under pressure to fix the spacers in the patterned photosensitive resin film.
According to the above-mentioned method, the alignment film is formed on the patterned photosensitive resin film on which the spacers are dispersed. Therefore, where a convex plate for printing is pressed against the substrate with the spacers dispersed thereon so that the alignment film is coated thereon by offset printing and flexographic printing; in some cases, spacers peel off from the electrode substrate to adhere to the convex plate.
Furthermore, when another alignment film is coated onto the other electrode substrate, the spacers adhering to the convex plate are transferred to this alignment film. The transferred spacers are not allowed to adhere to the electrode substrate, so that such spacers peel off in a rubbing step, a washing step, or the like, conducted later. In particular, in the case where the spacers are transferred onto pixel regions, regions where the transferred spacers peel off will have no alignment films.
More specifically, pinholes are formed in the alignment films and when a liquid crystal display device is completed, liquid crystal molecules cannot be aligned in the pinhole portions of the alignment film, resulting in a display defect and decrease in the number of good quality products.
Furthermore, during the rubbing step, the spacers adhering to the electrode substrate come into contact with a rubbing cloth. Therefore, the surface of the rubbing cloth coming into contact with the spacers becomes deformed. The alignment films rubbed with the deformed portion have inconsistencies in tilt angle. This causes a display defect when a liquid crystal device is completed.
Furthermore, the spacers adhering to the regions other than the pixels sometimes peel off from the substrate during rubbing. When rubbing is continued under this condition, the alignment film has scratches 10 as shown in FIG. 11, causing a display defect and decrease the number of good quality products.
According to the method utilizing static electricity as disclosed in Japanese Laid-Open Patent Publication No. 4-321013, spacers are charged and non-electrode portions are charged to a polarity opposite to that of the spacers, whereby the spacers are allowed to adhere to the non-electrode portions.
However, since the spacers dispersed in a hermetically sealed device fail to adhere to the electrode substrates, when the difference in electric potential between the spacers and the non-electrode portions is small the spacers also adhere to the pixel electrodes as shown in FIG. 12. Thus, when a liquid crystal display device is completed, light leaks through the spacers present on the pixel electrodes, leading to a decrease in contrast.
In contrast, in the case where the difference in electric potential between the spacers and the non-electrode portions is large; when a liquid crystal display device is completed, a display defect attributable to the charged spacers and non-electrode portions occurs and the number of good quality products decreases. Furthermore, in active-type liquid crystal display devices, there is a possibility of damage to active elements.
Furthermore, since the spacers do not adhere to the substrates, the spacers move due to the vibration and change in temperature during injection of liquid crystal or after a panel is constructed. As a result, inconsistencies in color resulting from inconsistencies in cell gap occurs. Inconsistencies in display occurs because the spacers damage the alignment films while moving.
According to the method for fixing spacers in a black mask as disclosed in Japanese Laid-Open Patent Publication No. 6-43468, dispersibility of the spacers in a resin material for a black mask is poor and the spacers are poorly dispersed as shown in FIG. 13, when a liquid crystal display device is completed by forming the black mask under this condition. Thus, inconsistencies in cell gap occur and a display becomes unsatisfactory, leading to decrease in the numbers of good quality products.
Furthermore, when the spacers are mixed in the resin material for the black mask and a mixture is allowed to stand, the spacers aggregate to further worsen the dispersibility of the spacers. When a liquid crystal display device is completed, inconsistencies in cell gap occur and a display becomes unsatisfactory, resulting in decrease in the number of good quality products. In ordinary wet dispersion, a dispersion liquid with spacers mixed therein is dispersed with stirring in order to prevent the spacers from aggregating. It is difficult to form a black mask while stirring the resin material for the black mask.
The spacers start aggregating by the time the resin material for the black mask is coated and cured on the electrode substrates. That is, in the case where the spacers are mixed in the resin material for the black mask, when the resin material has low viscosity, the dispersibility of the spacers right after being mixed in the black mask is satisfactory. However, the spacers start aggregating by the time when the liquid is coated and cured on the electrode substrates. In contrast, when the viscosity of the resin material for the black mask is high, the spacers are not likely to aggregate by the time when the liquid is coated and cured on the electrode substrates; however, the dispersibility of the spacers right after being mixed in the black mask becomes unsatisfactory.
Since alignment films are formed after the spacers are fixed, in the case where the alignment films are coated by offset printing or flexographic printing; the spacers sometimes peel off from the electrode substrates when a convex plate is pressed against the electrode substrates and the peeled spacers adhere to the convex plate, causing the same problems as those in the method utilizing a photosensitive thermoplastic resin disclosed in Japanese Laid-Open Patent Publication No. 2-308224.
Furthermore, during the rubbing step, a rubbing cloth comes into contact with the spacers adhering to the electrode substrates, so that the surface of the rubbing cloth which comes into contact with the spacers is deformed, causing the same problems as those in the method disclosed in Japanese Laid-Open Patent Publication No. 2-308224.